building an am receiver

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amriths04

Joined: 15 Jul 2006
Posts: 241
Helped: 13

24 Nov 2006 15:30 bc107 simple receiver

hello friends,
i just wanted to get your advice before i can try this out.
i intend to build a simple am receiver in a "BREAD BOARD".

my idea is to first have an
1)antenna(about 1m)
2)tuned circuit(L || C(var)).
3)this output is given to a double stage (bc 107 bjts), with gain=100*100=10000 so that it is amplified well.
4)this amplified output is then given to an envelope detector.(using a germanium diode and R and C combination).
5)this output is then sent to another single stage amplifier.
6)the output is taken from speakers.

my doubt is,whether my logic is correct? is there any practical problem in the above steps? or any other alternative?

thanking you in advance,
amrith.s

Borber

Joined: 01 Jan 1970
Posts: 1561
Helped: 117
Location: Slovenia

24 Nov 2006 19:41 Re: building an am receiver

Your concept is usuall for beginners level. Pay attention on impedance matching between LC tank and following amplifier. Bandwidth and selectivity of the receiver depends on Q factor of the LC tank. You can find some examples of such receiver on internet.

flatulent

Joined: 19 Jul 2002
Posts: 4875
Helped: 324
Location: Middle Earth

24 Nov 2006 20:44 Re: building an am receiver

This is a TRF (tuned rf) type receiver which was popular in the 1920s.

artem

Joined: 22 May 2003
Posts: 1652
Helped: 91
Location: Turan

24 Nov 2006 21:19 Re: building an am receiver

amriths04 wrote:

hello friends,
...
3)this output is given to a double stage (bc 107 bjts), with gain=100*100=10000 so that it is amplified well.
...
amrith.s

could be hard to achieve.

amriths04

Joined: 15 Jul 2006
Posts: 241
Helped: 13

25 Nov 2006 3:51 Re: building an am receiver

sorry borber,
i cannot understand how to match the impedance between the tuned circuit and the amplifier. would you kindly recommend me a site which deals with impedance matching between stages?

thank you,
amrith.s

Borber wrote:

Added after 2 minutes:

hello artem,
do you mean i should not use bc 107 or that i should not use bjt itself?
then what other device i can use for the rf amplifier stage? can i use a high frequency opamp? please advise.

thank you,
amrith.s

artem wrote:

amriths04 wrote:

hello friends,
...
3)this output is given to a double stage (bc 107 bjts), with gain=100*100=10000 so that it is amplified well.
...
amrith.s

could be hard to achieve.

Added after 2 minutes:

this is the circuit i designed.
please advise.

thank you,
amrith.s

Sorry, but you need login in to view this attachment

flatulent

Joined: 19 Jul 2002
Posts: 4875
Helped: 324
Location: Middle Earth

25 Nov 2006 4:52 Re: building an am receiver

You are using a power diode for the detector. The capacitance may be too high. Try using a small signal diode like the 1N918 silicon type or the 1N5711 Schottky type.

budhy

Joined: 21 Oct 2006
Posts: 836
Helped: 185
Location: Indonesia

25 Nov 2006 5:35 Re: building an am receiver

You are using strange power supply connection on you circuit, please check again.

You don't use a capasitor on small signal common emitter amplifier

Last edited by budhy on 25 Nov 2006 6:54; edited 1 time in total

Borber

Joined: 01 Jan 1970
Posts: 1561
Helped: 117
Location: Slovenia

25 Nov 2006 6:10 Re: building an am receiver

Your design is totally wrong.
LC parallel tank connected in series with antenna is bandstop filter. It must be connected as bandpass filter.
Amplifier is wrong, it's gain is about 0dB or one, not 10000 what you expect. This is because transistors are saturated. The ratio of collector to emitter resistors is correct but 330k collector resistor is too large. Start design with assumption of collector current of 1-2mA. Transistors should operate in A class. And at the end calculate bias resistors.
Visit:
http://www2.eng.cam.ac.uk/~dmh/ptialcd/trf/trf.htm

amriths04

Joined: 15 Jul 2006
Posts: 241
Helped: 13

25 Nov 2006 9:40 Re: building an am receiver

ya, i understood that.
can you just say whether the below design would be right. pardon me, i am still a beginner..

let Vcc=10v,
let ic=1mA,
let Rc=5.6k,
thus, Vc=10-5.6=4.4v,

gain=100,
thus, Re=0.056k,
thus,Ve=0.056v,
and Vb=0.656v.

so,biasing resistors, Ra(top one)=93.4k and Rb=6.6k.
using another stage with similar configurations will make it a double stage with gain 100*100. will this work?
and how to change that LC ckt into a bandpass arrangement?

thankyou,
amrith.s

Borber wrote:

Added after 2 minutes:

ya budhy, i got what you meant. it was a mistake which i commited while drawing it in pspice.
which capacitor you mean to say?

amrith.s

budhy wrote:

You are using strange power supply connection on you circuit, please check again.

You don't use a capasitor on small signal common emitter amplifier

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Borber

Joined: 01 Jan 1970
Posts: 1561
Helped: 117
Location: Slovenia

25 Nov 2006 14:51 Re: building an am receiver

Your amplifier will have gain of about 30. Try to get some info here:
http://my.integritynet.com.au/purdic/index.html#rf-amplifier-design

Are you using pspice only for drawing schematics? It can simulate your circuit and that is the main purpouse of spice programs.

amriths04

Joined: 15 Jul 2006
Posts: 241
Helped: 13

25 Nov 2006 15:42 Re: building an am receiver

but i am having a student version of pspice. i donno how to use it for transient analyses. do you have any tutorial on pspice?

Borber wrote:

Your amplifier will have gain of about 30. Try to get some info here:
h**p://my.integritynet.com.au/purdic/index.html#rf-amplifier-design

Are you using pspice only for drawing schematics? It can simulate your circuit and that is the main purpouse of spice programs.

Borber

Joined: 01 Jan 1970
Posts: 1561
Helped: 117
Location: Slovenia

25 Nov 2006 16:10 Re: building an am receiver

Pspice shurelly have all basic simulations like AC and transient. I do not use pspice and can not help you directly. Try to use Help or search internet for pspice manual. You will find a lot of informations.
Multisim or Electronics Workbench is more suitable for beginners I think.

flatulent

Joined: 19 Jul 2002
Posts: 4875
Helped: 324
Location: Middle Earth

25 Nov 2006 20:11 Re: building an am receiver

Your receiver needs a means to change the gain at the input of the audio amplifier.

amriths04

Joined: 15 Jul 2006
Posts: 241
Helped: 13

26 Nov 2006 2:04 Re: building an am receiver

do you mean i need to use an agc in there or just a voltage divider with a nominal gain would do?

thank you,

flatulent wrote:

Your receiver needs a means to change the gain at the input of the audio amplifier.

flatulent

Joined: 19 Jul 2002
Posts: 4875
Helped: 324
Location: Middle Earth

26 Nov 2006 2:38 Re: building an am receiver

Between the detector and the audio amplifier have a variable resistor with one end grounded. The far end goes to the detector and the variable tap goes to the audio amplifier.

Some other things to watch out for is oscillation of the two stage RF amplifier. This is an issue of careful layout and power supply filtering.

You may also want to have a switch between two antennas, one very long and one very short. That way local stations will not clip in the RF stage.